The use of SQUIDS as sensors to measure magnetic fields or small voltages is well known. SQUID sensors have generally been of two types commonly referred to as RF SQUID and dc SQUID. The RF SQUID utilizes a single Josephson junction in a closed superconductive loop. The dc SQUID requires two Josephson junctions connected in a closed superconductive loop with a dc bias connected across the two junctions in parallel. RF SQUIDS have been constructed by forming a toroidal chamber with a small gap left in the central post of the toroid. The Josephson junction is formed by a pointed screw bridging the gap and forming a weak conductive link. One or more coils wound from superconducting wire within the toroidal chamber magnetically couple to the current circulating in the closed loop through the junction. While such a device can be built with a high coefficient of coupling and with very low inductance formed by the SQUID body as well as relatively high coil inductance to match well to current source impedances, this design does not lend itself to use with dc SQUIDS. While in principle it is possible to use point contact, weak link Josephson junctions in a dc configuration, it is impractical to adjust the two junctions accurately enough to achieve the uniformity of design required for a commercially acceptable product.
For this reason, dc SQUIDS in the past have employed Josephson junctions using thin film semi-conductor junctions. By using semi-conductor techniques and technology, which is now highly developed, repeatability and fabrication can be more readily achieved. However, because of their inherent planar geometry, SQUID devices using thin film junctions do not lend themselves to obtaining a high coefficient of coupling, low inductance in the junction loop, and high inductance in the coupling coil that is desired.